Structure, and method and apparatus for founding a structure

ABSTRACT

A method of founding a structure in a subaqueous bed comprising disposing the structure on the bed and excavating beneath the structure, the excavation being effected by a wheeled positively buoyant vehicle which moves upon an undersurface of the structure and which is provided with excavating equipment.

Jan. 8, 1974 United States Patent [191 Hansen S T w T MM s E m T mm .MSe um n N U m u STRUCTURE, AND METHOD AND APPARATUS FOR FOUNDING ASTRUCTURE 538,073 4/l895 Harris..................... [75 Inventor: FrodeJohan Hansen, Kingswood, 3,683,632 8/1972 Van der England 3,218,73911/1965 Kaufmann et a1. 3,629,963 12/1971 FOREIGN PATENTS ORAPPLICATIONS [73] Assignee: Redpath Dorman Long (Contracting) Limited,Edinburgh, Scotland 2,021,441 3/1971Germany...............................61/50 Primary Examiner-JacobShapiro Attorney-Bacon & Thomas 22 Filed: Feb. 4, 1972 21 Appl. No.:223,590

[57] ABSTRACT A method of founding a structure in a subaqueous bed [30]Foreign Application Priority Data Feb. 8, 1971 GreatBritain..................... 4,191/71 comprising disposing the Structureon the bed and cavating beneath the structure, the excavation beingeffected by a wheeled positively buoyant vehicle which moves upon anundersurface of the structure and which is provided with excavatingequipment.

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10 Claims, 16 Drawing Figures PATENTEU N 8 74 SHEET 2 BF 7 PATENTED JAN31974 SHEET 0F 7 mm Qmms mm 3 QR wE STRUCTURE, AND METHOD AND APPARATUSFOR FOUNDING A STRUCTURE This invention relates to a structure adaptedto be founded on a subaqueous bed. It also relates to a method offounding a sinkable structure on a subaqueous bed, and to a vehicle forexcavating a subaqueous bed under such structure.

In one aspect, to which the invention is not in general limited, thereis provided a vehicle for excavating under a structure disposed on asubaqueous bed, the vehicle being adapted to have positive buoyancy andcomprising means whereby the vehicle may contact andmove about on anundersurface of the structure, and means for supporting excavatingequipment.

The vehicle may be provided with powered wheels or rollers to enable itto contact and move about on the said undersurface.

The wheels or rollers may be arranged to permit the vehicle to move inany direction on the said undersurface and to rotate about its owncentre.

The vehicle may be provided with excavating equipment comprising acutter or breaker head, and/or means for providing a high pressure waterjet and/or a suction pipe, the suction pipe preferably also beingadapted to deliver material to the excavated space for back filling.

The vehicle may comprise a buoyancy chamber and means for admitting andexpelling water from the chamber to control the buoyancy of the vehicle.

The vehicle may be adapted to carry a crew, or alternatively the vehiclemay be provided with remote control equipment so that no crew isnecessary.

In another respect, to which the invention'is not in general limited,there is provided plant comprising a vehicle as set forth above, inconjunction with apparatus for moving the vehicle into a position fromwhich it can move into contact with the said undersurface of astructure.

There may be means for measuring the position and heading of the vehiclerelative to the apparatus.

The means for measuring may be adapted to measure the radial distance ofthe vehicle from the apparatus and the bearing of the apparatus relativeto the vehicle.

The said apparatus may consist of a pontoon adapted to be disposed aboveand in contact with the vehicle whereby the vehicle and pontoon areadapted to be submerged together and to move vertically whilstsubmerged.

There may be means for varying the buoyancy of the pontoon. i

The pontoon and the vehicle may be adapted to be disposed in a verticalshaft in the said structure, the pontoon being provided with means forsealing the space between its periphery and the wall of the shaft.

The pontoon may be provided with means for engaging the wall of theshaft whereby to vertically locate the pontoon relative to the shaft.

The vehicle and the apparatus each may include a pressure chamber, therethen being means to permit personnel to pass between the pressurechambers whilst both chambers are submerged.

In another aspect to which it is not in general limited the inventionprovides a sinkable structure adapted to be founded on a subaqueous bed,and comprising an undersurface adapted to contact the subaqueous bed,the structure being in combination with apparatus for moving a vehicleas set forth above into a position in which it can move into contactwith the undersurface and excavate the subaqueous bed beneath thestructure.

The last mentioned apparatus for moving may be as set forth above.

The apparatus may be a moveable portion of that part of the structurewhich defines the undersurface.

The structure may have a shaft extending upwards from the undersurfaceand adapted to have the vehicle and the apparatus moveably disposedtherein.

There may be means (e.g., the aforementioned means for sealing) forsealing the shaft so that water may be pumped out of the excavated bedbeneath the undersurface.

There may be a diving bell for communicating between the pressurechamber of the apparatus and a part of the structure which is above thewater surface when the structure has been sunk to the subaqueous bed.

The structure may be in two separate parts; a foundation part definingsaid undersurface and having a substructure which is adapted to projectabove the water surface when the structure has been sunk, and asuperstructure part which is adapted to be supported above water levelby the substructure.

In a further aspect, to which it is not in general limited, theinvention provides a method of founding a structure in a subaqueous bedcomprising disposing the structure on the bed and excavating beneath thestructure, the excavation being effected by a positively buoyant vehiclewhich moves upon an undersurface of the structure and which is providedwith excavating equipment.

The method may include excavating under the structure so that it issupported on a number of spaced apart regions (e.g., at four corners)and then increasing the effective load on the said regions so that thestructure sinks into the bed.

When the structure has a recess (e.g., a shaft) extending upwardly fromthe undersurface, the vehicle preferably retires to the recess whilstthe structure is sinking.

The effective load may be increased by sealing the excavated spacebeneath the undersurface and by pumping out water therefrom. t

The excavated space may be filled with solid material after thestructure has been sunk into the bed to a desired extent.

An embodiment of the invention will be described merely by way ofexample with reference to the accompanying drawings wherein FIG. I showsasinkable structure according to the invention in a buoyant condition,

FIG. 2 shows the structure of FIG. 1 having been sunk,

FIG. 3 is a vertical section through FIG. 2 with some parts added,

FIG. 4 and 5 are vertical sections through part of the foundation raftof FIG. 3, showing the excavating vehicle in two different positions,

FIG. 6 is a section on line Vl-Vl of FIG. 4

FIGS. 7d and 7b are respectively top and bottom plan views of theexcavating vehicle,

FIG. 8 is a vertical section through the foundation raft of FIG. 2,showing excavation proceeding,

FIG. 9 is a half-section on line IX-IX of FIG. 8,

FIGS. 10, 11 and 12 show successive stages in the founding of thefoundation raft, the figures being vertical sections as FIG. 8,

FIG. 13 is a section similar to that of FIG. showing an alternativeembodiment of the invention,

FIG. 14 shows the founded sinkable structure with its permanentsuperstructure positioned prior to being fitted, and

FIG. 15 shows the permanent superstructure in its final positionattached to the sinkable structure.

When building a structure which when completed is founded in asubaqueous bed such as a seabed or lakebed, it is desirable toprefabricate as much as possible of the structure on-shore, and to do aminimum amount of work at sea. Furthermore, the work at sea should be ofshort duration and of a simple nature and it should not be dependent onweather conditions or require expensive floating equipment.

The illustrated embodiments of the present invention are thought to meetthese desiderata. Referring to FIGS. 2 and 15, there is shown anoff-shore drilling rig having a foundation raft 20, a sub-structure 22and a super-structure 24 (FIG. 2) or 26 (FIG. 15).

The foundation raft is shown resting on the sea-bed 27 in FIG. 2, and inits finally dug-in or founded condition in FIG. 15.

The super-structure 24, 26 is supported at a safe height above sea leveland its exact nature depends on its purpose. Thus, in FIG. 15 thesuper-structure 26 is equipped with a drilling rig 28. In FIG. 2 thesuperstructure 24 is temporary only and is designed for use in foundingthe foundation raft in the sea-bed.

The sub-structure 22 supports the super-structure from the foundationraft 20 and comprises two rows of circular vertical steel columns 32,free at the top and suitably braced against each other at their lowerparts which are submerged when the structure is on-site.

The upper part of the sub-structure has a minimum area exposed to waveaction in order to reduce waveinduced forces to a minimum.

The lower part of the sub-structure is less subject to wave-inducedforces and so is not designed to present a minimum surface area. Insteadit is designed to have adequate buoyancy to ensure that the combinedsubstructure and foundation raft has adequate stability for towing tothe site.

The foundation raft 20 consists of a steel casing 34 (see FIG. 3), thebottom of which is reinforced with concrete 36 to prevent puncturing ofthe casing should it sink onto an uneven sea-bed and also to provide alow centre of gravity so that the foundation raft and sub structure arestable when floating, and during sinking. The casing 34 is divided intoa number of water tight chambers 38 which can be wholly or partiallyflooded to control sinking of the structure. When the chambers 38 arenot fully flooded, the foundation and substructure have sufficientbuoyancy and are stable enough to be towed to the site withoutadditional buoyancy aids being required.

The foundation raft 20 is provided with a flat undersurface 40 and aconcrete-reinforced shaft 42 extending upwardly from the surface 40.These features are of importance during the founding of the raft 20 inthe sea-bed, and will be discussed later.

The foundation raft 20, the sub-structure 22 and the temporarysuperstructure 24 are constructed into their complete state onshore. Thesubstructure is provided on top of the columns 32 with lifting tackle(sheaves) 44, and the temporary superstructure 24 is provided withwinches 46. Cables or chains 48 pass around the lifting tackle 44 andthe winches 46.

When the structure is to be towed to the site where it is to operate,the superstructure 24 is temporarily supported as shown in FIG. 1 uponupper members 49 of the cross-bracing structure between the columns 32.

The structure is then towed in any suitable manner whilst floating asshown in FIG. 1.

When the structure arrives at the site, the temporary structure hoistsitself upwards by winching-in the cables or chains 48, until it is inthe position shown in FIG. 2. It will be noted that the hoistingoperation is continuous rather than a discontinuous climbing operation.Also, the lifting tackle 44 and cables 48 do not transmit bendingmoments due to wave action from the columns to the super-structure 24.

The foundation raft 20 is then partially flooded so that it sinks in acontrolled manner to the sea-bed 27, as shown in FIG. 2. The process offounding the raft 20 in the seabed then is effected, by means ofequipment 'now to be described. This equipment permits the foundationraft 20 to be sunk into the sea-bed without using normal compressed airmethods, which would limit the possible water and foundation depth.

The equipment comprises a crewed vehicle 50 adapted to have positivebuoyance and which during sinking of the raft 20 is disposed in theshaft 42 beneath a pontoon 52. The pontoon 52 has a controllablepositive and negative buoyancy.

Referring to FIGS. 4, 5 and 6 the pontoon 52 comprises a pressurechamber 54 adapted to contain a crew and ventilated via an air line 56.The chamber 54 may also be pressurised for use as a decompressionchamber, should it be necessary for the crew of the vehicle 50 to bedecompressed. Surrounding the chamber 54 is a buoyancy chamber 58 whichmay be flooded to a controlled extent to control thebuoyancy of thepontoon. An inflatable seal 60 is provided for sealing the space betweenthe shaft 42 and the pontoon, for reasons discussed hereafter.

A diving bell 62 is arranged on a pulley system 64 to provide fortransport for men from the chamber 54 to the surface. An air-lock 66permits men to move between the chamber 54 and the diving bell 62.

An an alternative to the diving bell 62 and the airlock 66, there may beprovided a temporary access shaft between the chamber 54 and thesuperstructure 24. However, such a shaft would be subject to waveinducedforces and the diving bell is considered simpler and safer.

The pontoon 52 is provided with three hydraulically operated pins 68(FIG. 6) which may be extended to engage in recesses 70 in the wall ofthe shaft 42 whereby-to fix the pontoon in the shaft 42. The recesses70, are long slots permitting the pontoon to be fixed in any positionbetween the position shown in FIG. 4 and the position shown in FIG. 5and discussed hereafter.

Surrounding the chamber 72 is a buoyancy chamber 78 which in operationis subjected internally and externally to water pressure. This chambermay be flooded as necessary to control the buoyancy of the vehicle 50.When in operation the chamber 78 is not flooded so that the vehicle 50has a positive buoyancy of several tons.

The vehicle 50 is provided in its upper part with wheels or rollerswhereby it may move about on the under surface (see FIG. 7a) thefoundation raft 20. In this embodiment (see FIG. 7a) there are one pairof oppositely-disposed powered wheels 80 and two spherical free-wheels82. The wheels are arranged about a common centre coinciding with theaxis of the vehicle 50, so that the vehicle can rotate about its ownaxis and move in any direction. The wheels 80 are powered by hydraulicpressure fluid motors shown diagrammatically at 84. The pressure fluidsource for these motors is provided on the super-structure 24 and fedvia lines (not shown) to the vehicle 50. Alternatively, the pressurefluid may be provided by an electrically driven pump in the chamber 78,Control of the motor is effected from within the chamber 72. Instead ofhaving wheels 80, 82 the vehicle may have tracks which pass around idlerwheels and drive sprockets.

The vehicle 50 is-provided with a pivotally mounted tool holder 86 towhich may be attached a variety of cutting or breaking tools, eitherrotary or percussive. A rotary tool is shown fitted at 88. It will beseen from FIG. 7 b that the tool holder 86 is pivotable in a verticaldiametral plane of the vehicle 50. Hydraulic working fluid for the tool(provided from the superstructure 24) is fed down the interior of thetool holder.

Another similarly pivotably mounted tool holder 90 may support aninterchangeable nozzle 92, to provide a high-pressure, soil-breakingwaterjet. A downwardlydepending suction nozzle 94 also is provided forremoving loosened soil. The suction is provided via a pipe 96 from apump on the superstructure 24. The pump is reversible so that the pipe96 and nozzle 94 may deliver suspended solid material for backfilling,as described later.

The vehicle has access hatches 98, 100 between the pressure chamber 72and the buoyancy chamber 78, and between the buoyancy chamber 78 and theoutside of the vehicle 50.

These hatches permit the crew to service or change the tool 88, thenozzle 92 and the suction nozzle 94 by pressurising the chamber 72 andthen emerging into the buoyancy chamber 78. Alternatively the suctionnozzle 94, which is flexible, may be pulled in to the chamber 78 forservicing (e.g., if it is blocked). The need for servicing can be seenby watching the discharge from the pipe 96.

When the foundation raft 20 is sunk, the pontoon 52 and the vehicle 50are within the shaft 42 as shown in FIG. 3. To commence the foundingoperation, water is drained from the air-lock 75 into the chamber 54.This forces the top of the vehicle 50 into sealing contact with thebottom of the air-lock 75, permitting two men hicle 50 will sink.Resting on the sea-bed, the vehicle 50 digs itself in as shown in FIG. 4until the pontoon 52 has sunk so deep that its underside is flush withthe undersurface of the raft 20 as shown in FIG. 5. At that point thepins 68 are engaged with the recesses 70 in the shaft wall and thepontoon 52 cannot sink any deeper. The vehicle continues diggingunderneath itself until there is a clearance underneath it equal to thereach of its digging equipment 86, 90 and the suction pipe 94.

At that stage power is supplied to the wheels 80 and the vehicle 50moves away sideways from the pontoon onto the flat undersurface 40 ofthe raft 20 as shown in FIG. S'and starts digging whilst moving beneaththe raft 20 around the pontoon 52 in bigger and bigger circles untilultimately the raft is resting on four spaced-apart triangular cornerareas 101, as shown in FIGS. 8 and 9.

The digging is effected by breaking up soft soil with the high pressurewater jet 92, and harder material by means of the tool 88. The broken-upmaterial is removed via the suction pipe 94. Boulders may be broken upby means of a heavy drop-chisel (not shown in the drawings, butconventional in itself). Limited areas of rock may be dealt with bymeans of a rock-breaker (not shown) projecting from the side of thevehicle 50.

The most effective method of digging is to apply downward forces to thematerial to be removed. Then the undersurface 40 of the raft 20 providesa firm support for the vehicle 50 enabling the reaction force on thevehicle 50 to be absorbed without upward movement of the vehicle, whichwould reduce the effectiveness of the digging operation.

If the vehicle has to excavate against a vertical face, the lower partof the face is attacked with downward forces to produce local slips. Theloose material falls beneath the vehicle and is removed via the suctionpipe. There is no danger to the vehicle 50 because the slip tends topush it away from the face.

During the excavating process boulders that are too large to be removedby the suction pipe may collect on the floor of the excavation. Thesemay have to be broken up by the drop-chisel if they impede movement ofthe vehicle.

As the excavation proceeds, the foundation raft 20 will sink, and theextent of the sinking is constantly checked by monitoring the clearancebetween the vehicle 50 and the floor of the excavation. A similar checkis effected from the chamber 54 of the pontoon 52.

to pass via the hatches 76, 77 from the chamber 54 to l the chamber 72.The water pressure in the airlock 75 is then re-established The buoyancyof the pontoon 52 is adjusted from the pontoon pressure chamber 54 byletting some air out of the buoyancy chamber 58 which automatically letswater in and at some point the pontoon 52 and the ve- A preferablemethod of combined excavation and sinking is for the vehicle to retireperiodically to the shaft 42, and for the weight of the raft 20 to beincreased by admitting more water to the chambers 38. The raft thensinks whilst the vehicle 50 is safely in the shaft 42, the pontoon 52having raised itself to the FIG. 4 position so that the vehicle can beaccommodated.

As the digging proceeds, there will ultimately be a stage in which theraft 20 is level and has sunk so deep that the excavated space is sealedfrom the external water around the perimeter of the raft 20. The vehicle50 then moves onto the undersurface of the pontoon 52 and the pontoonand vehicle together move into their FIG. 4 position.

The chambers 38 of the foundation raft 20 are then completely flooded toincrease the penetration into the sea-bed. The seal is inflated and thesuction pipe 94, 96 is used to pump water out of the excavated space 102(FIG. 10), more quickly then it can permeate into the space 102 throughthe surrounding strata. The pressure in the space 102 is thus reducedand the downward force exerted by the raft 20 on the comer supports 101is greatly increased, (perhaps by as much as four times) and the raftsinks deeper into the sea-bed if the corner supports 101 cannotwithstand the increased downward force.

The operation can be repeated if necessary by deflating the seal 60,equalising the pressure inside the space 102 to the external waterpressure and using the vehicle 50 to excavate more material from beneaththe raft. Some of the water can be pumped out of the chamber 38 tofurther lighten the load.

Ultimately, the reduced water pressure in the space 102 and thecorresponding temporary increase of the load on the supports 101 doesnot produce any further settlement of the structure, and it then followsthat the four corner supports 101 and the friction on the side walls ofthe raft 20 can carry the structure with an ample factor of safety,because the temporarily increased load on the supports greatly exceedsthe weight of the structure.

Then there is no need to dig the raft 20 in any deeper and the vehicle50 can now finalise the operation by backfilling the space 102underneath the raft with sand and gravel 104 (FIG. 11) which furtherincreases the safety factor of the foundation. The sand and gravel areobtained from elsewhere on the sea-bed and are deliv ered via thesuction pipe 94, 96. The extremities of the space 102 are filled bydirecting the sand and gravel by means of the high pressure jet 92. Ifnecessary the vehicle 50 may move out onto the undersurface of the raft20 to expedite the backfilling operation.

When the backfilling operation is complete and the shaft 42 has beenfilled as much as possible, (FIG. 12), the pins 68 of the pontoon arewithdrawn from their slots 70 in the shaft wall. The buoyancy of thepontoon 52 and the vehicle 50 is adjusted to be slightly positive, andthe pontoon and vehicle rise to the surface, their work completed.Alternatively, the buoyancy can be adjusted to be only slightlynegative, the vehicle and the pontoon then being hauled to the surfaceby a winch on the superstructure 24.

The vehicle 50 can be made remotely controllable from the temporarysuperstructure 24, thus making it unnecessary for the vehicle to becrewed. When remotely controlled, the vehicle is provided withclosedcircuit television cameras and sufficient external lighting for anoperator on the superstructure 24 to control the digging operation. Theequipment for controlling the movements of the vehicle and the operationof its various excavating equipments 86, 90, 94 may be designed for eachparticular vehicle according to conventional hydraulics practice.

Since the vehicle is not crewed, the pressure chamber 72 need not beprovided. The pressure chamber 54 in the pontoon 52, the diving bell 62and the associated air-locks 75, 66 also can be dispensed with. Thepontoon may merely be a concrete slab, effectively a moveable portion ofthe bottom of the foundation raft 20. In such a case, the inflatableseal 60 may be carried in a frame attached to the concrete slab.

By remotely controlling the vehicle, it becomes necessary for men towork under water only in an emergency or if the excavating equipment 86,90, 94 needs servicing.

A remotely controlled vehicle is shown at in FIG. 13. Parts shown inthis figure and already described with reference to other figures carrythe same reference numerals as in those figures.

The vehicle 150 operates in conjunction with a flatbottomed concreteslab 152 which serves as a pontoon. The slab 152 may be raised andlowered by cables 156. Downward movement of the slab 152 is limited byprojection 157 arranged so that when the slab is in its lowest positionits undersurface is flush with the undersurface 40 of the raft 20.

Compressed air is supplied via a line 158 to the interior of the vehicle150. The air pressure is controlled by a float valve 159 operating in astand-pipe 160 open to external water pressure so that the pressuresinside and outside the vehicle 50 are roughly equal.

The vehicle 50 has a television camera 162. In order that the operatoron the super-structure 24 may know the position of the vehicle, agyrocompass 164 indicates the angular orientation or heading of thevehicle, and a distance meter 166 indicates the radial distance of thevehicle from the centre of the pontoon slab 152.

The distance meter consists of a wire 168 attached to the centre of thepontoon slab 152 at 170 and extending through a guide fuse 172, over atensioning roller 174 to a drum 176. The guide tube is pivotally mountedabout a vertical axis in the roof of the vehicle 50. The length of cableunwound from the drum 176 indicates the distance of the vehicle from thepoint 170, and the angular position of the guide tube 172 indicates thebearing of the point 170 relative to the vehicle 50. These twovariables, together with the gyrocompass output, completely define theposition and heading of the vehicle 50.

Similar position and heading finding equipment may be provided on thecrewed vehicle 50 if required.

A telescopic hydraulic jack 178 is provided for sampling levelling orplate-testing the sea-bed.

It will be appreciated that with both the crewed and remotely controlledversion of the vehicle, it is possible to establish the exact positionand orientation of the vehicle, and to view the progress of the diggingoperation beneath the raft20. i

When the foundation raft 20 has been dug into the sea-bed to therequired level and the chamber 38 fully flooded, the structure should besafe against any likely weather conditions. When the temporarysuperstructure 24 is removed the only exposed structure are the tops ofthe columns 32 with the lifting tackle 44 thereon; all the cross-bracingis now below the surface because the foundation raft 20 has sunk intothe sea bed.

The permanent super-structure 26 leaves the fabrication yard as acompletely self-contained buoyant seaworthy unit with all the requiredequipment and facilities installed'and in working order.

It is towed to the site and when the tide and weather conditions aresuitable it is moved into position between the columns 32, as shown inFIG. 14.

Heavy winches 108 on the super-structure 26 are utilised with the lightlifting tackle 44 to hoist heavy lifting tackle to the tops of thecolumns 32, the heavy tackle being shown installed at 110 in FIG. 14.

Heavy chains and cables 112 are passed around the lifting tackle 110 tothe winches 108, and by means of these the super-structure 26 liftsitself from the water. When it reaches the required level, permanentmountings for the super-structure 26 are substituted for the liftingtackle 110. These permanent mountings consist essentially of brackets114 (FIG. which contact the tops of the columns 32 via rollers 116, sothat bending moments due to wave action are not transmitted from thecolumns to the superstructure.

It will be appreciated that the full carrying capacity of the supportingcolumns is established before any permanent super-structure load isapplied, and the lifting starts immediately when load is applied to thecolumns.

The lifting operation is one continuous hoisting operation and not adiscontinuous climbing up the supporting columns, as witha jack-upplatform.

The lifting operation is completely independent of any wave or swellaction as soon as the superstructure is clear of the water, and neitherduring the lifting nor in its final position will the superstructure besubject to additional bending stresses arising from wave action on thesubstructure.

I claim:

1. A sinkable structure adapted to be founded on a subaqueous bed, andcomprising an undersurface adapted to contact the subaqueous bed, incombination with a vehicle forexcavating under the structure beingfounded on the subaqueous bed, the vehicle comprising means forproviding positive buoyancy, means enabling the vehicle to contact andmove about on the undersurface of the structure, and means forsupporting excavating equipment; and an apparatus for moving the vehicleinto a position in which it can move into contact with the undersurface,and excavate the subaqueous bed beneath the structure.

2. A structure as claimed in claim 1 wherein the apparatus comprises avariable-buoyancy pontoon.

3. A structure as claimed in claim 1 wherein the apparatus is a moveableportion of that part of the structure which defines the undersurface. Y

4. A structure as claimed in claim 1 wherein the structure has a shaftextending upwards from the undersurface and adapted to have the vehicleand the apparatus moveably disposed therein.

5. A structure as claimed in claim 4 provided with means for sealing theshaft so that water may be pumped out of the excavated bed beneath theundersurface.

6. A method of founding a structure having an undersurface in asubaqueous bed, comprising the steps of disposing the structure on thebed moving a positively buoyant vehicle provided with excavatingequipment into contact upon the undersurface of the structure, movingsaid vehicle about on the undersurface of the structure, and excavatingthe subaqueous bed beneath the structure with said excavating equipment.

7. A method as claimed in claim 6, comprising excavating under thestructure so that it is supported on a number of spaced apartnonexcavated regions and then increasing the effective load on the saidregions so that the structure sinks into the bed.

8. A method as claimed in claim 7, wherein the vehicle retires to arecess in the structure extending upwardly from the undersurface whilstthe structure is sinking.

9. A method as claimed in claim 7 wherein the effective load isincreased by sealing the excavated space beneath the undersurface and bypumping out water therefrom.

10. A method as claimed in claim 6 wherein the excavated space is filledwith solid material after the structure has been sunk into the bed to adesired extent.

1. A sinkable structure adapted to be founded on a subaqueous bed, andcomprising an undersurface adapted to contact the subaqueous bed, incombination with a vehicle for excavating under the structure beingfounded on the subaqueous bed, the vehicle comprising means forproviding positive buoyancy, means enabling the vehicle to contact andmove about on the undersurface of the structure, and means forsupporting excavating equipment; and an apparatus for moving the vehicleinto a position in which it can move into contact with the undersurface,and excavate the subaqueous bed beneath the structure.
 2. A structure asclaimed in claim 1 wherein the apparatus comprises a variable-buoyancypontoon.
 3. A structure as claimed in claim 1 wherein the apparatus is amoveable portion of that part of the structure which defines theundersurface.
 4. A structure as claimed in claim 1 wherein the structurehas a shaft extending upwards from the undersurface and adapted to havethe vehicle and the apparatus moveably disposed therein.
 5. A structureas claimed in claim 4 provided with means for sealing the shaft so thatwater may be pumped out of the excavated bed beneath the undersurface.6. A method of founding a structure having an undersurface in asubaqueous bed, comprising the steps of disposing the structure on thebed moving a positively buoyant vehicle provided with excavatingequipment into contact upon the undersurface of the structure, movingsaid vehicle about on the undersurface of the structure, and excavatingthe subaqueous bed beneath the structure with said excavating equipment.7. A method as claimed in claim 6, comprising excavating under thestructure so that it is supported on a number of spaced apartnonexcavated regions and then increasing the effective load on the saidregions so that the structure sinks into the bed.
 8. A method as claimedin claim 7, wherein the vehicle retires to a recess in the structureextending upwardly from the undersurface whilst the structure issinking.
 9. A method as claimed in claim 7 wherein the effective load isincreased by sealing the excavated space beneath the undersurface and bypumping out water therefrom.
 10. A method as claimed in claim 6 whereinthe excavated space is filled with solid material after the structurehas been sunk into the bed to a desired extent.